As is well known, metal structures which must be immersed in electrolytes, such as iron or iron alloys in water, are subject to a significant problem of corrosion. This problem occurs due to the flow of local current through the electrolyte between localized cathodic and anodic portions of the immersed structure.
Accordingly, in the past, a number of systems have been developed to counteract this corrosion by making the metal structure to be a protected part of an electrical arrangement which holds the structure at a predetermined electrical potential. This is accomplished by providing an electrode which is also immersed in the electrolyte and spaced apart from the structure to be protected. The structure and the electrode are then coupled to terminals of differing potentials of a direct current voltage source (typically a rectified alternating current voltage). Thus, the structure forms part of a circuit comprised of the voltage source, the electrode, the electrolyte, and the structure itself. This provides a predetermined polarity of potential to the structure to reduce the likelihood of the development of corrosion produced by local current flow.
If the structure is maintained at a negative potential, the system is known as a cathodic protection system. On the other hand, if the structure is maintained at a positive potential, the system is known as an anodic protection system. Of course, in either case, the electrode will have a polarity opposite to that of the structure. Further, in either case, it is desirable to maintain the structure at a predetermined potential to continue to prevent corrosion, since an improper potential level can reduce the effectiveness of corrosion prevention, and, in some cases, actually stimulate the corrosion.
For illustrative purposes, all discussion hereinafter will relate to a cathodic protection system wherein the structure to be protected is a cathode, and the electrode with which it cooperates is an anode. Of course, it is understood that the system discussed herein can readily be converted to an anodic protection system by reversing the polarities of the structure and the electrode.
To hold the structure at a desired potential level, e.g. at a predetermined negative potential for cathodic protection, it is necessary to sense the potential of the structure and make any changes necessary to the rectified alternating current source for correcting the current flow between the cathode and the anode to maintain the desired potential level. Typically, the sensing is done through the use of a reference cell (e.g. a standard Cu-CuSO.sub.4 half cell) which is also located in the electrolyte spaced apart from the structure. This reference cell is coupled to a measuring means such as a voltmeter. The measuring means is also coupled to the structure. Therefore, the measuring means can readily determine the potential of the structure. Since it is known that the potential of the structure has to remain at a certain level to be effective to prevent corrosion, the determination of the structure's potential allows control of the voltage applied between the anode and the structure which establishes the potential of the structure. Such control can be manual, or with an automatic adjuster for the rectified alternating current source coupled to the anode and cathode.
A problem which develops in these prior art systems is error in the measurement taken by the measuring means due to the effect of the so-called IR drop. This IR drop is developed by current produced between the anode and the structure. This current develops a voltage V.sub.IR at the reference cell proportional to the current between the anode and cathode multiplied by the resistance of the electrolyte and structure coatings. Obviously, as the resistivity of the electrolyte or coating increases, this voltage component V.sub.IR becomes more significant. And, since it is not a proper variable for determining the potential of the structure via the reference cell, it can become a source of significant measurement error.
This problem becomes more serious when the reference cell is located close to the anode. Accordingly, attempts have been made in the past to reduce V.sub.IR by locating the reference cell near or even on the structure itself as far away as possible from the anode. However, with this arrangement, the reference cell has a reduced portion of structure which it can effectively monitor. Further, it may actually block out a portion of the structure which it is adjacent to, thereby preventing maintenance of the proper negative potential at the blocked-out portion.
One prior art system which seeks to reduce this problem is taught in U.S. Pat. No. 3,425,921 issued to Sudrabin on Feb. 4, 1969. This system uses a pair of reference cells immersed in an electrolyte rather than the conventional single reference cell. This pair of reference cells is located so that the potential drop between one of the reference cells and the structure is greater than the potential drop of the other. These reference cells then form part of a bridge circuit such as shown in FIG. 2 of that patent for controlling the rectified alternating current source. This arrangement allows a reading of the desired polarization potential of the structure without interference from the IR drop. Although this arrangement does reduce measurement error caused by the IR drop, it is somewhat inconvenient in requiring the use of an extra reference electrode for each measuring location.
Other prior art systems have also been developed to offset the effect of the IR drop. For example, in U.S. Pat. No. 3,634,222 issued to Stephens, Jr. on Jan. 11, 1972, the current between the anode and the cathode is turned off each time a reference reading is to be taken. In U.S. Pat. No. 4,080,272 issued to Ferry et al on Mar. 21, 1978, the reference reading is taken at a zero direct current output point of the full-wave rectified alternating current. These systems also improve the measurement accuracy. However, they both require a specific timed relationship between the current flow between the anode and the cathode and the reference reading taken which, in turn, requires a somewhat complicated circuit arrangement to maintain this specific timed relationship.